The Raman spectra of mixed crystals of [(NH₄)_1−x K_x]₂ SO₄ in the region 50–3400 cm⁻¹ at 293 K and below 223 K have been reported. At room temperature 293 K, as the concentration of K⁺ ion increases in the crystal up to 50%, the frequencies of the totally symmetric vibrations of SO₄²⁻ and NH₄⁺ ions increase and thereafter the frequency of SO₄²⁻ vibration decreases and attains the value in K₂SO₄. This change in frequency up to 50% of potassium concentration is due to the breaking of hydrogen bonds of the type N-H...O. The behaviour of Raman intensities of A_g(v₁) mode of SO₄²⁻ for various concentrations (x=0, 0·03, 0·11, 0·5, 0·85) suggest that the phase transition changes from first order type to one of second order. The phase transition in mixed crystals of [(NH₄)_1−x K_x]₂ SO₄ can be a cooperative phenomenon arising from a coupling between (NH₄)⁺ ions through hydrogen bonds with the distorted SO₄²⁻ ions in the low temperature phase.